Electromagnetic radiation in the infra-red region (IR radiation) can be a useful tool for obtaining information about an object when visible light is low or non-existent or when it is absorbed or scattered by the atmosphere. In the former case, there may be insufficient visible light to provide a reflected visible image of the object; but IR radiation is directly emitted from objects in amounts varying with their temperature regardless of the amount of visible light present for reflection from the objects. In the latter case, there are several wavelength bands within the total IR spectrum which are transmitted with relatively less loss and/or scatter through fog, mist and similar atmospheric conditions which scatter and/or absorb visible light. In either case, moreover, IR radiation provides information relating to the temperature of objects: information which is useful for the differentiation of objects in a scene but is not available at the shorter wavelengths of visible light.
Use of this tool in scanning a scene, however, has been hampered by the expense and difficulty of detecting the IR radiation. Generally, detecting an image of a scene requires scanning an array of detectors, serially detecting the IR radiation falling on each individual element of the detector array and reporting out that information to the user in some appropriate format. Such an array of IR detectors is quite expensive: prohibitively so for many consumer applications. In addition, such detectors typically require the use of materials with which it is difficult to provide uniformity of response from one detector to the next. This produces an undesirable skewing of information from different parts of the scene. Furthermore, many such detectors are thermally activated and require their input to be chopped so as to provide a thermal recovery period between sequential readings. Such chopping has traditionally been provided by moving optical apparatus, such as spinning or oscillating mirrors, placed in the optical path to the detector(s); and this apparatus further increases the expense and mechanical complexity of the apparatus.